Quantum tunneling, magnetization temperature

While this generally has only small effects on the static magnetic properties, it may play a relevant role in providing a quantitative agreement with experimental resonance data, and in explaining quantum tunnelling relaxation of the magnetization at low temperature [51]. 

For the partially deuterated benzoic acid (C6D5COOH), the solid state H NMR spectrum is dominated by the intra-dimer H- H dipole-dipole interaction. In a single crystal, both tautomers A and B are characterised by a well-defined interproton vector with respect to the direction of the magnetic field (Fig. 1). Proton motion modulates the H- H dipole-dipole interactions, which in turn affects the H NMR lineshape and the spin-lattice relaxation time. It has been shown that spin-lattice relaxation times are sensitive to the proton dynamics over the temperature range from 10 K to 300 K, and at low temperatures incoherent quantum tunnelling characterises the proton dynamics. A dipolar splitting of about 16 kHz is observed at 20 K. From the orientation dependence of the dipolar splitting, the... 

However, the tunneling effects are also evident at higher temperature in what has been called thermally activated quantum tunneling of the magnetization. The states higher in the... 

At t=T the escape rate begins to depart from the above reported law, tending to a temperature-independent quantum transition rate (Fig. G.l), which has the same form, in the usual WKB (Wentzel-Kramers-Brillouin) approximation for quantum tunneling, as in the case of switching of magnetization via thermal activation ... 

Results in favor of the existence of macroscopic quantum tunneling of the magnetization were also reported by means of low-temperature measurements of the frequency-dependent magnetic noise S co) and... 

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